It is now well-established that oligosaccharides play an important biological role especially as regards to the activity and function of proteins; thus, they serve to modulate the half-life of proteins, and occasionally they are involved in the structure of the protein. Oligosaccharides play an essential role in antigen variability (for example blood groups), and in certain bacterial infections such as those caused by Neisseria meningitidis. 
As oligosaccharides are usually obtained in a low yield by purification starting from natural sources, the synthesis of oligosaccharides has become a major challenge of carbohydrate chemistry. In particular, it is a goal to supply sufficient amounts of well-characterized oligosaccharides, required for fundamental research or for any other potential applications.
The synthesis of complex oligosaccharides of biological interest may be performed chemically, enzymatically or microbiologically. Despite the development of new chemical methods for synthesizing oligosaccharides in the course of the last 20 years, the chemical synthesis of oligosaccharides remains very difficult on account of the numerous selective protection and deprotection steps, the lability of the glycoside linkages, the difficulties in obtaining regiospecific couplings, and the low production yields.
GD3 (Neu5Acα-8Neu5Acα-3Galβ-4GlcCer) is a minor ganglioside found in most normal tissues in higher vertebrates including humans. The GD3 level has been shown to increase during some pathological situations, such as cancers (glioma, melanoma) and to have an important role in tumor angiogenesis Zeng, et al. Cancer Res, 60:6670 (2000). Anti-GD3 monoclonal antibodies have been shown to inhibit the growth of human melanoma cells both in vitro and in vivo (Birkle, et al. Biochimie, 85, 455 (2003); Ruf, et al. Int J Cancer, 108, 725 (2004)). In normal cells, GD3 is a cell death effector, activating the mitochondrial-dependent apoptosome in response to apoptotic stimuli (Fernandez-Checa, Biochem Biophys Res Commun, 304, 471 (2003)). In addition, GD3 has a proapoptotic function by suppressing the nuclear factor-Kb-dependent survival pathway (Colell, et al. Faseb J, 15, 1068 (2001)).
Chemical synthesis of the oligosaccharide moiety of gangliosides is difficult to achieve (Castro-Palomino et al., Chemistry, 7, 2178 (2001)) but new efficient biotechnological techniques have recently been developed for the synthesis of GM3, GM2 and GM1 oligosaccharides (Priem et al. Glycobiology, 12, 235 (2002) and Antoine et al., Chembiochem, 4, 406 (2003)). The GM3 oligosaccharide (Neu5Acα-3Galβ-4Glc) was synthesized by a metabolically engineered Escherichia coli strain which overexpressed the Neisseria meningitidis genes for α-3 sialyltransferase and CMP-Neu5Ac synthase. Lactose and neuraminic acid (Neu5Ac) were supplied as exogenous precursors and actively internalized by E. coli 's β-galactosidase and Neu5Ac permease. To prevent catabolism of the precursors, a mutant strain devoid of both β-galactosidase and Neu5Ac aldolase activities was used.
Despite advances in the art, new biosynthetic methods for producing desired oligosaccharides are needed. The present invention addresses these and other needs.